Hi All. In the great earthquake of April 1906 near Daly City (there is some debate about this location) the fault ruptured and propagated out both North and South where the displacement was up to 21 feet in the Point Reyes area, the largest surface displacement ever recorded. Along the San Francisco Peninsula section of the fault the displacement was less, and it dropped yet again South of Black Mountain. Some geophysicists speculate that this might have happened because the moving rupture partially dissipated when it hit the obstruction.

On the theory that the great 1906 earthquake did not dissipate much of the accumulated strain South of Black Mountain as it presumably did North of San Francisco. The USGS in 1988 assigned a relatively high risk (30% over the next 30 years of a major earthquake along the Santa Cruz Mountains section of the San Andreas, between Black Mountain and San Juan Bautista. This was the highest risk assigned to any fault segment in the Bay Area. (Ward and Page, 1989)

As we all know we had the Loma Prieta quake in 1989. The northward portion of the rupture didn’t quite make in to Black Mountain. The reason is not fully understood but it is believed that because the fault makes a slight right hand jog towards the north this stopped the rupture from reaching Black Mountain.

This leaves the area between most northern end of the Loma Prieta ruptured to an area somewhere near Daly City that has not any significant quake activity since the 1906 quake. This area in one of two known seismic gaps along the San Andreas fault. The other one is south of Parkfield.

In the past 5 days starting on 06/04/02 to 06/09/02 there have been 9 small quakes in the area of Pacifica, California. During the same time period there was a small quake at the entrance to the San Francisco Bay about 5 miles north of Daly City. On 06/09/02 there was a 1.3Md quake 5 miles south of Portola Valley. This is also south of Black Mountain. From 1912 to 1990 there have been 36 quakes within a 5-mile radius of this quake. The smallest quake not counting this one was a M>1.5 and largest was a M>4.5. There have also been a small number of quakes about 5 miles north of Portola Valley. None that I can remember were over M>3.0. That quake occurred in 1999. There have been more since then, but are not listed in the NEIC catalog.

In the past there have been small quake swarms in the area of Pacifica, but to have a quake occur along the San Andreas fault near the southern end of the seismic gap during the same time period is something new. At least to me it is as this is the first time I have seen this combination occurring at the same time.

As to what this means for the future at this time, is hard to say, but I feel that if more activity is seen between Portola Valley and Pacifica it could be a good indication that the area is close to breaking. How close is only a guess and one that I’m not ready to make. There is still to little to go on. Take Care…Don in creepy town